A machine tool having a numerical control device incorporated therein performs machining on a workpiece while moving a movable unit to move a table on which the workpiece is mounted, or on a tool or the like. Meanwhile, a machine tool including a rotation axis can control a tool attitude relative to a machining workpiece by rotating the rotation axis. For example, the machine tool of this type often machines a plurality of machining surfaces having different normal vectors on the machining workpiece or cuts the machining surfaces themselves. When the machine tool machines machining surfaces having different normal vectors, a conventional numerical control device designates a corresponding coordinate system to each of the machining surfaces. That is, the numerical control device expresses an origin (a machining reference point), based on which a command position such as a tip position of the tool is determined by a machining program or a manual feeding device, and a coordinate axis by translation or translational motion and rotation of an arbitrary reference coordinate system (see, for example, Patent Literature 1).
Furthermore, in the case of a numerical control device according to another conventional technique, an operator of the numerical control device designates a coordinate system suitable for machining whenever switching a plurality of machining surfaces. That is, it takes the operator time and labor to, for example, select and designate a coordinate system suitable for machining from among preset coordinate systems.